The invention relates to a method of increasing the service life of grinding wheels which are intended for grinding the borders or edges of spectacle lenses. Each wheel comprises a core wheel, preferably made of plastic, and a ring around the core made of sintered metal with embedded diamond particles.
A grinding wheel of this type is described in German Utility Model 72 02 327. The core wheel made of plastic is fastened on a grinding spindle by a hub bore. A ring attached to this core wheel consists of sintered bronze in which fine diamond particles are embedded. Grinding wheels of this type can be used to grind to shape the borders of spectacle lenses made of silicate glass and plastic to enable these lenses to fit into a selected spectacle frame. Usually, a grinding wheel of this type is also provided with a beveling groove in order for a ridge bevel to be formed on the ground-to-shape spectacle lens. The ridge bevel is used for retaining the spectacle lens in a groove of the spectacle frame.
Although known grinding wheels with a ring made of sintered metal and embedded diamond particles are comparatively costly, they have a long service life, provided the ring made of sintered metal and the core wheel made of plastic are produced from suitable materials and are subjected to stringent quality control. The grinding wheel undergoes non-uniform wear during grinding of spectacle lenses. Dressing and evening renders the grinding wheel usable again until the sintered metal ring has become so thin that any further use is no longer possible.
The dressing operation is necessary after approximately 5000 spectacle lenses made of silicate glass have been ground. Although the grinding wheel is evened again during the dressing, so much abrasive material has to be removed from the surface of the grinding wheel for dressing it that its service life is limited overall. To remedy this disadvantage, U.S. Pat. No. 4,233,784 proposes a grinding method which can supposedly be used to grind 25,000 or more spectacle lenses made of silicate glass. For this purpose, the grinding wheel is scanned before the grinding operation. The spectacle lens is then positioned on a region of the grinding wheel which is higher than adjacent regions. As a result, grinding of a spectacle lens always subjects the higher regions of the grinding wheel to wear, and a longer service life can be achieved overall without the grinding wheel having to be dressed. The control means used for this purpose do not increase the costs of the grinding machine to any significant extent.
German Offenlegungsschrift 1 502 438 discloses a grinding apparatus in which feeding of a flushing medium releases the particles which are embedded in the surface and flushes these particles out of the surface. In this case, the flushing medium is repeatedly deflected between the surfaces of the wheel and the baffle plate. As a result, that medium should repeatedly come into close contact with the surface of the wheel. This sufficiently cools the surface and the accumulated abraded material is released.